Cyclic Nucleotide-Gated Ion Channel 2 modulates auxin homeostasis and signaling

Cyclic Nucleotide Gated Ion Channels (CNGCs) have been firmly established as Ca ²⁺ -conducting ion channels that regulate a wide variety of physiological responses in plants. CNGC2 has been implicated in plant immunity and Ca ²⁺ signaling due to the autoimmune phenotypes exhibited by null mutants of CNGC2 . However, cngc2 mutants display additional phenotypes that are unique among autoimmune mutants, suggesting that CNGC2 has functions beyond defense and generates distinct Ca ²⁺ signals in response to different triggers. In this study we found that cngc2 mutants showed reduced gravitropism, consistent with a defect in auxin signaling. This was mirrored in the diminished auxin response detected by the auxin reporters DR5::GUS and DII-VENUS and in a strongly impaired auxin-induced Ca ²⁺ response. Moreover, the cngc2 mutant exhibits higher levels of the endogenous auxin indole-3-acetic acid (IAA), indicating that excess auxin in cngc2 causes its pleiotropic phenotypes. These auxin signaling defects and the autoimmunity syndrome of cngc2 could be suppressed by loss-of-function mutations in the auxin biosynthesis gene YUCCA6 ( YUC6 ), as determined by identification of the cngc2 suppressor mutant repressor of cngc2 ( rdd1 ) as an allele of YUC6 . A loss-of-function mutation in the upstream auxin biosynthesis gene TRYPTOPHAN AMINOTRANSFERASE OF ARABIDOPSIS ( TAA1, WEAK ETHYLENE INSENSITIVE8 ) also suppressed the cngc2 phenotypes, further supporting the tight relationship between CNGC2 and the TAA–YUC-dependent auxin biosynthesis pathway. Taking these results together, we propose that the Ca ²⁺ signal generated by CNGC2 is a part of the negative feedback regulation of auxin homeostasis in which CNGC2 balances cellular auxin perception by influencing auxin biosynthesis.